UWB transceivers generally operate in the range of 3.1 to 10.6 GHz. As known, UWB frequency has a range of 3.1-4.8 GHz and is divided into several bands. When there are three carrier frequency bands, the first band may be 3432 MHz, the second may be 3960 MHz and the third one may be 4488 MHz carrier frequency, with a channel spacing of 528 MHz. p Clock generators are an essential part of a wireless transmission system. A clock generator is used to generate different carrier frequencies which are required to modulate different aspects of channel information with multiplexing. One of these schemes is used in an UWB system where the channel information is modulated with OFDM (Orthogonal Frequency Division Multiplexing). For OFDM and other applications, it is required to generate a clean clock signal (with very low phase noise/jitter) for modulation of the data. A clock generator may use a PLL (Phase Locked Loop) and a VCO (Voltage Controlled Oscillator).
Generally, the required carrier frequencies are generated by PLLs. A PLL is a major component in the frequency synthesizer scheme and uses a wide, flexible range of internal frequency dividers which allow the designer the ability to create a frequency synthesizer to match design requirements.
Some essential performance requirements for general RF wireless transceiver systems include:                1. Low phase noise        2. Low cost (power, area)        3. Low settling time/less hopping time between different carrier frequencies        4. Low spurious/harmonic signals        5. Stable outputs in spite of process/temperature/voltage variations        6. VCO of a clock generator PLL should work at a frequency which is higher than the required carrier frequencies to avoid “VCO pulling effect” due to any of these carrier frequencies, which may cause false locking of the VCO.For an UWB system Band-A, the required carrier frequencies are:            a. 3432 MHz    b. 3960 MHz    c. 4488 MHz
These frequencies are hopped within a very short duration (<300 ns) due to symbol switching for OFDM transmission, with the result that a very small portion of this time interval (<9 ns) is available for stabilization of the carrier frequency (one out of these 3 at a time). Owing to these fast hopping requirements, all the 3 frequencies should be available simultaneously, and a frequency multiplexer is used to select one out of the three carrier frequencies.
A conventional method to generate 3 carrier frequencies is to use 3 different PLLs and multiplex the outputs of these PLLs. These PLLs need to work at twice the frequencies to meet requirement #6 as mentioned above, and these frequencies are then divided by 2 to generate the required frequencies.
The disadvantages of these prior art schemes include:
1. Higher cost (3 PLLs consume a large amount of power, and area).
2. More closed loop blocks, more issues related to stability.
3. Difficulties of implementation, as all the PLLs are working at high speeds.
It is desirable to address the foregoing disadvantages and provide an improved system and method for generating carrier frequencies required for wireless transmission applications.